What is needed would vary from state to state. At bare minimum, I think all the states require that you have a marriage license, someone who has the authority to conduct the ceremony, and two witnesses. Some states have additional requirements, such as having blood tests or being a minimum age. (I believe the additional requirements are prerequisite to obtaining the marriage license.)
Of course, since Metropolis is in the fictitious state of New Troy, you are free to add or subtract requirements as you see fit.
Joy,
Lynn
